The present invention is principally useful in on-site sewage disposal systems which are comprised of septic tanks and leaching fields. The invention is an improvement to the leaching systems which have heretofore been known.
Generally, sewage is digested in a vessel commonly called a septic tank and liquid is discharged therefrom to be dispersed and renovated by bacterial action and oxidation beneath the surface of the earth. Such dispersions may be accomplished in a variety of ways, but most commonly the effluent is run through perforated piping placed within stone filled leaching trenches dug within the native earth material.
Typically, a trench is partially filled with stone, a perforated pipe is laid therein, additional stone is added, the stone is covered with a semi-permeable membrane such as building paper or salt hay, and the membrane is covered with earth. A common alternative method is to place large pre-cast concrete structures, called galleries, within larger trenches with and without the use of surrounding stone. Galleries operate in analogous fashion to the pipe system but they typically have larger volume to surface area ratios.
The stone in the leaching trenches serves the purposes of supporting the distribution pipe and the sides and top of the soil cavity. The effluent discharge surface area is commonly considered to be the area of the sides and bottom of the stone-soil interface. The pore volume within the stone mass, commonly about one third of the bulk volume of one inch nominal diameter stone, provides storage capacity for surges in liquid flow. The pore volume also establishes the effective area at the soil-stone interface.
However, there are problems associated with the use of stone. These include that it is often costly to obtain in requisite quality, is labor intensive to install, and tends to be installed by heavy equipment in a way which compacts and decreases the permeability of the soil. Since stone occupies about two-thirds of the earth-cavity volume, it thereby substantially limits the desired liquid storage volume. And, over time the stone can be infiltrated by and filled with fine soil, thus degrading both its storage capacity and the effluent dispersal surface area which was originally contemplated.
Thus, there have been developed and are used certain "no-stone" leaching systems. But these devices have limitations which the present invention is intended to overcome. Principal among the no-stone systems are the previously mentioned galleries, or precast concrete leaching chambers, such as shown in U.S. Pat. No. 3,645,100 of LaMonica. Typically, such chambers may measure 4 by 8 by 1.5 feet in dimension. Not only are such units costly but they require large earth openings and heavy equipment to install. Furthermore, the typical discharge area achievable in precast concrete units tends to be about 8 percent of the chamber sidewall, and thus they can only be used without stone in certain suitable soils. There is also marketed a perforated pipe of relatively large (8-10 inch) diameter having a filter fabric on the exterior. Fine filter media risks the propensity for clogging over time and damage during installation.
Thus, common problems with prior art no-stone systems are both the limited discharge area into the earth and blockage of the effluent holes.
Various patents show considerable past effort on alternatives for dispersing or collecting liquids in the earth and some have bearing on the present invention. U.S. Pat. No. 460,352 to Reading shows a perforated drain tile with lips over the perforations in the pipe sidewall. Auriemma U.S. Pat. No. 4,183,696 shows a plastic drainage pipe having triangular shaped openings with a protective lip over the openings.
Gutman in U.S. Pat. No. 2,366,522 shows an arch shaped leaching unit for burying in the earth, but the effluent can only flow vertically downward. See also Nicholson U.S. Pat. No. 2,866,319 and Olsen U.S. Pat. No. 3,440,823 where there are small exit holes at the bottom of the arch. Flynn in U.S. Pat. No. 3,579,995 shows an rectanguloid cross section open-bottomed concrete unit with holes in the sidewalls. A series of different extension lips over each horizontal row of holes is said to keep the sides of the earth trench stable and spaced apart from the buried walls of the unit.
However, of the various units mentioned above, only the pre-cast concrete galleries have met with both widespread commercial use and regulatory agency approval for septic tank leaching systems. Principally, this appears to be due to the failure of the other units to demonstrate requisite functionality and durability. Most probably, this is because with the passage of time the surrounding soil tends to move against the sidewalls and thus limit the efflux of liquid.
Thus, there remains a need for a leaching system which is light in weight and easily installed in a variety of soils, and which maintains its capacity for exfiltration over a period of time.